With recent expansion of use of optical fibers in subscriber systems, there is a persisting and stringent demand for reduced size and cost of optical modules. In order to meet for such a stringent demand, it is necessary to simplify the connection between an optical semiconductor device and an optical waveguide or optical fiber in the optical module, such that the optical connection is achieved easily. In the same context, it is necessary to simplify the construction of the optical module and an optical module assembly, such that the optical semiconductor devices can be integrated with a large integration density.
FIGS. 1A shows an example of a conventional optical module 101 of the type that is adapted for receiving an incident optical beam at a rear side of a photoreception device.
Referring to FIG. 1A, the optical module 101 includes a support substrate 1 of Si that holds thereon an optical fiber 20. The support substrate 1 further supports thereon a sub-mount 2 typically formed of a ceramic material in a vertical state, wherein the sub-mount 2 supports thereon a photoreception device 201 such that a photoreception region of the device 201 faces an edge surface 20A of the optical fiber 20.
The photoreception device 201, on the other hand, includes a device substrate 3 of n-type InP, wherein a buffer layer 4 of n.sup.+ -type InP and an optical absorption layer 5 of undoped InGaAs are formed consecutively on the device substrate 3 by a planar process. On the optical absorption layer 5, a layer 6 of n.sup.- -type InP is formed, while the InP layer 6 thus formed is further formed with p-type InP regions 7a and 7b as a result of diffusion of Zn into the layer 6 in correspondence to the foregoing regions 7a and 7b. The diffusion regions 7a and 7b are then covered by p-type ohmic electrodes 8a and 8b respectively, wherein each of the ohmic electrodes 8a and 8b is formed by a deposition of an Au/Zn/Au stacked structure, followed by an annealing process thereof.
The InP layer 6 of the n.sup.- -type is covered by a thin insulation film (not shown) of SiN or SiO.sub.2, and an insulation layer 12 of a polyimide is formed further on the insulation film. The insulation layer 12 as well as the insulation film underneath are then formed with contact holes respectively in correspondence to the foregoing p-type diffusion regions 7a and 7b, and metal bumps 9a and 9b, each having a Ti/Pt/Au/Sn/Au stacked structure or a Ti/Pt/AuSn stacked structure, or alternatively of a Ti/Pt/Sn/Au stacked structure, are formed respectively on the p-type diffusion regions 7a and 7b. Thereby, the foregoing p-type electrodes 8a and 8b are formed in contact with the metal bumps 9a and 9b respectively. Further, the device substrate 3 is formed with a microlens 10 by an etching process on a rear side of thereof in correspondence to the p-type diffusion region 7b. The microlens 10 thus formed is covered by an anti-reflection coating 3b.
The sub-mount 2, on the other hand, carries thereon conductor patterns 2c and 2d, and the foregoing metal bumps 9a and 9b of the photoreception device 201 are flip-chip mounted on the conductor patterns 2c and 2d respectively. It should be noted that the conductor pattern 2c is connected to a conductor pattern 2a on the rear side of the sub-mount 2 by way of a via-hole 2b extending through the sub-mount 2, and a positive dc voltage E is applied to the conductor pattern 2a. On the other hand, the conductor pattern 2d is connected to a conductor pattern 2f on the foregoing rear side of the sub-mount 2 by way of a via-hole 2e extending also through the sub-mount 2, and a load R.sub.L is connected to the conductor pattern 2f.
The optical module 101 is thus formed by mounting the sub-mount 2 on the support substrate 1 such that the photoreception device 201 establishes an optical alignment with a core 20a of the optical fiber 20.
FIG. 1B shows an equivalent circuit diagram of the photoreception device 201.
Referring to FIG. 1B, the photoreception device 201 includes two pin diodes D1 and D2 formed respectively between the n-type buffer layer 4 of InP and the p-type diffusion region 7a of InP and between the n-type buffer layer 4 of InP and the p-type diffusion region 7b of InP, wherein the buffer layer 4 is common to the pin diode D1 and the pin diode D2.
In the circuit of FIG. 1B, the diode D1 is biased in the forward direction and acts as a current source. The p-type diffusion region 7a typically has a very large area (300.times.200 .mu.m, for example) and supplies a large drive current. While the diode D1 has a relatively large junction capacitance Cp of about 6 pF, for example, this large junction capacitance is tolerable for a current source.
On the other hand, the diode D2 is reverse-biased and acts as a photodiode forming the photoreception region. Thus, the optical beam emitted from the edge surface 20A of the optical fiber 20 is focused on the part of the optical absorption layer 5 where the diode D2 is formed by the microlens 10. In response to such an exposure to the optical beam, electron-hole pairs are excited in the optical absorption layer 5 efficiently, and the electrons and holes thus excited cause a drifting in respective, opposite directions in accordance with the electric field induced between the p-type electrodes 8a and 8b. In other words, a photo-current flows through the diode D2. As the p-type diffusion region 7b has a very small size, typically smaller than about 40 .mu.m in diameter, the junction capacitance of the diode D2 is also very small, typically about 0.15 pF. Thus, a very high response is obtained at the load R.sub.L for the optical detection.
On the other hand, the optical module 101 of FIG. 1B has a drawback, associated with the complex support structure of the photoreception device 201, in that the proper alignment of the photoreception device 201 with respect to the optical waveguide or optical fiber 20 is difficult. While it is true that the sub-mount 2 itself, mounted directly on the support substrate 1, may be positioned on the support substrate 1 by using a marker M formed on the support substrate 1, the desired proper positioning of the photoreception device 201 with respect to the optical fiber 20 is still difficult due to the mounting error between the photoreception device 201 and the sub-mount 2. In the module 101 of FIG. 1A, therefore, it has been necessary to adjust the position of the optical fiber 20 on the support substrate 1 with a sub-micron precision by monitoring the output of the photoreception device 201 while applying a bias thereto.
FIG. 2 shows another conventional optical module 102 that also receives an incident optical beam at a rear side of a photoreception device.
Referring to FIG. 2, the photoreception module 102 includes an optical waveguide 13 on the support substrate 1, and a photoreception device 202 is held above the support substrate 1 by means of an intervening sub-mount 14. The sub-mount 14 includes an oblique surface, and the oblique surface reflects an incident optical beam toward the rear side of the photoreception device 202.
It should be noted that the optical waveguide is formed of glass or semiconductor layers formed on the support substrate 1 of Si by way of a CVD process and has a structure in which a core layer 13b having a refractive index n.sub.1 is sandwiched vertically or laterally by claddings 13a and 13c each having a refractive index n.sub.2 smaller than the refractive index n.sub.1.
The photoreception device 202 has a structure generally identical with that of the photoreception device 201 except that the microlens 10 is omitted. Further, the photodetection region D2 corresponding to the p-type diffusion region 7b has a slightly increased size in correspondence to the increased beam size of the incident optical beam, which may be a slightly divergent optical beam.
It should be noted that the sub-mount 14 has mutually parallel upper and lower major surfaces, and holds the photoreception device 202 above the support substrate 1 as noted previously. Further, the sub-mount 14 includes an oblique surface at the right side wall thereof with an oblique angle .THETA. with respect to the support substrate 1, wherein the oblique surface is covered by a reflective metal coating 14a. Thereby, the optical beam emitted from an edge surface 13A of the optical waveguide 13 is reflected by the foregoing oblique surface 14a and reaches the optical absorption layer 5 forming the diode D1 after entering the rear surface of the photoreception device 202.
In the optical module 102, it should be noted that the positional relationship between the optical waveguide 13 and the support substrate 1 is fixed. No degree of freedom exists here. Further, the photodiode D2 of the photoreception device 202 has a relatively large area for receiving the incident optical beam. Thus, the optical module 102 can be fabricated relatively easily, without monitoring the photoreception output current, by using a marker provided on the support substrate 1. However, the foregoing optical module 102 that uses an additional, intervening sub-mount 14, has a drawback in that the number of the parts in the module is increased and the mounting process is complicated.
Thus, the foregoing prior art devices, each using an intermediate sub-mount 2 or 14 in combination with a photoreception device 201 or 202, both of which being of the type that receives the incident optical beam at the rear substrate surface, have suffered from the problem of increased number of parts, complex process of assembling the optical module, difficulty in reducing the size, and the like.
On the other hand, there is further proposed an optical module 103 shown in FIG. 3A, in which the optical module 103 includes a photoreception device 203 mounted on the support substrate 1 on which the optical waveguide 13 is formed monolithically, similarly to the optical module 102 of FIG. 2. In the optical module 103, the photoreception device 203 is flip-chip mounted on the support substrate 1 via a solder layer 19, wherein the photoreception device 203 is formed with a flat oblique surface 3A at the edge of the device substrate 3 so as to face the edge 13A of the optical waveguide 13 with an angle .THETA..sub.1. Thereby, the optical beam emitted from the edge surface 13A of the optical waveguide 13 is refracted by the oblique surface 3A toward the junction region 7b where the photodiode D2 is formed.
FIG. 3B shows another conventional optical module 104 that includes the optical waveguide 13 on the support substrate 1 monolithically similarly to the previous example of FIG. 3A, wherein the optical module 104 further includes a photoreception device 204 on the support substrate 1. The photoreception device 204 is formed on the device substrate 3 that has a vertical side wall with respect to the major surface of the support substrate 1, wherein the photoreception device 204 is disposed such that the foregoing vertical side wall faces the edge surface 13A of the optical waveguide 13. Thereby, the optical beam emitted from the foregoing edge surface 13A enters the device substrate 3 of the photoreception device 204 at the vertical side wall.
Further, the photoreception device 204 includes the oblique surface 3A generally at a center of the device substrate 3 with an angle .THETA..sub.2 with respect to the principal surface of the support substrate 1, wherein the oblique surface 3A of the device of FIG. 3B is covered by a reflective metal coating 15, such that the oblique surface 3A causes a reflection of the optical beam emitted from the edge surface 13A of the optical waveguide 13 and entered into the device substrate 3 at the vertical side wall, toward the diffusion region 7b. It is also possible to cause the desired deflection of the optical beam by the total reflection at the foregoing oblique surface 3A. It should be noted that the photoreception device 204 is mounted on the support substrate 1 by soldering the foregoing metal coating 15 covering the lower major surface of the device substrate 3 on the top surface of the support substrate 1.
In any of the foregoing conventional optical module structures of FIGS. 3A and 3B, the sub-mount 2 or 14 used in the previously described prior art is eliminated, and the number of the parts forming the module is reduced substantially. Further, the optical module structures of FIG. 3A or 3B, in which the photoreception device is mounted directly on the support substrate 1, allows an exact positioning of the photoreception device 203 or 204 on the support substrate 1 by merely using a marker M. Thus, the optical module 103 or 104 is easy for assembling, and the cost of the device is reduced substantially.
It should be noted that the construction of FIG. 2 as well as the constructions of FIGS. 3A and 3B are disclosed in the Japanese Laid-open Patent Publication 8-316506 corresponding to the U.S. patent application Ser. No. 08/552,474 filed Nov. 9, 1995, which is incorporated herein as reference. It should be noted that the foregoing Japanese Laid-open Patent Publication 8-316506 was laid-open on Nov. 29, 1996.
On the other hand, there is a further room for improvement in the optical modules 103 and 104 of FIGS. 3A and 3B with regard to size, integration density, easiness of assembling and cost.